1. Field of the Invention
The present invention relates to an image forming apparatus applied to electrophotography and electrostatic recording.
2. Description of the Related Art
As a developing method of visualizing an electrostatic latent image, electrophotography is most widely used. Of the electrophotographic known schemes, a two-component developing method using a two-component developing agent is generally employed. This developing agent is a mixture of a fine coloring powder called a toner, and a magnetic powder, called a carrier. In this method, toner particles are charged to be electrostatically attracted to a latent image.
In the two-component developing method, however, a developing unit tends to be increased in size. For this reason, in recent small copying machines and printers, one-component developing methods, which require no carrier, have been increasingly employed.
Of these one-component developing methods, a method of using a nonmagnetic toner facilitates a reduction in size, weight, and cost of a developing unit because it requires no expensive magnetic rollers. An advantage of another one-component developing method using an elastic roller as a developing roller is that even if the developing roller is brought into contact with an electrostatic latent image carrying member (a photo-conductive member in electrophotography), the electrostatic latent image carrying member is not damaged. Since the developing roller is brought into contact with the electrostatic latent image carrying member in this method, a developing electrode can be positioned near an electrostatic latent image. This improves the sharpness of character and line images, and hence allows development with high image quality.
In such a one-component developing method using a nonmagnetic toner, however, since a developing roller has no magnetic pole, nonmagnetic toner particles must be transferred by an electrostatic force and a physical force between the developing roller and the nonmagnetic toner. For this reason, if images are continuously formed, the following problems are posed: a decrease in black solid image density, the defective formation of a toner layer, an increase in background fog, and the like.